The present invention relates to a protective circuit for the input of a multimeter or similar instrument, and particularly to a circuit for protecting against higher than normal input voltages.
Protective circuits are suitably employed at the input of sensitive instruments to protect against voltages higher than normal operating voltages. One arrangement for providing this kind of protection comprises means in series with the instrument such as a fuse or thermistor, and a parallel protection component such as a spark gap, zener diode or varistor.
The combination of a series PTC (positive temperature coefficient) thermistor and a parallel MOV (metal oxide varistor) has proved particularly advantageous. The thermistor is characterized by low input impedance during normal operation but develops very high impedance when heated in response to excessive current. The shunt connected varistor causes transient overvoltages to bypass the protected circuit. Metal oxide varistors are faster than spark gaps and handle much higher energy than zener diodes, and when overloaded the metal oxide varistors fail in a shorted condition therefore continuing to provide circuit protection. In most cases the thermistor and varistor circuitry can recover spontaneously upon removal of the overload.
However, the above described circuit is limited not only in regard to the degree of steady state overvoltage protection but also with respect to transients. The series PTC thermistor is typically adapted to withstand voltages in the vicinity of 600 volts with complete recovery. However, it would be desirable for an instrument to withstand inadvertent application of higher voltages.
The shunt varistor is the first line of protection against transients but can be particularly susceptible to repetitive transients having peak amplitudes exceeding the varistor knee voltage although the rms values may not exceed the instrument's voltage rating. The varistor operates very rapidly upon the presentation of a high voltage transient and will conduct before the series connected thermistor senses an overload. Dissipation in the shunt connected varistor subjected to repeated transients can be many times the dissipation in the series thermistor, and unless the varistor is considerably oversized it can become permanently damaged.